Normal Iron Absorption and Storage
Iron facts
  • All body cells need iron. It is crucial for oxygen transport, energy production, and cellular growth and proliferation.
  • The human body contains an average of 3.5 g of iron (males 4 g, females 3 g).
  • The typical daily American diet contains 10–20 mg of iron.
  • Only about 10% of dietary iron is absorbed (1–2 mg/day).

Iron absorption

  • Iron is mainly absorbed in the duodenum and upper jejunum.
  • A protein called divalent metal transporter 1 (DMT1) facilitates iron transfer across intestinal epithelial cells.
  • Normally, individuals absorb less than 10% of dietary iron, or 1–2 mg per day balancing the daily loss from desquamation of epithelia.
  • Most absorbed iron is used in bone marrow for erythropoiesis.
  • Iron homeostasis is closely regulated via intestinal absorption.
  • Once iron is absorbed, there is no physiologic mechanism for excretion of excess iron from the body other than blood loss (i.e., pregnancy, menstruation or other bleeding.)

Iron transport

  • Most absorbed iron is transported in the bloodstream bound to the glycoprotein transferrin.
  • Transferrin is a carrier protein that plays a role in regulating the transport of iron from the site of absorption to virtually all tissues.
  • Transferrin binds only two iron atoms.
  • Normally, 20–45% of transferrin binding sites are filled (measured as percent transferrin saturation [TS]).

 

Normal Iron Absorption and Metabolism

illustration of normal iron absorption and metabolism

Iron is bound and transported in the body via transferrin and stored in ferritin molecules. Once iron is absorbed, there is no physiologic mechanism for excretion of excess iron from the body other than blood loss i.e., pregnancy, menstruation or other bleeding.

Iron use in the body

  • 75% of absorbed iron is bound to proteins such as hemoglobin that are involved in oxygen transport.
  • About 10% to 20% of absorbed iron goes into a storage pool that is also recycled in erythropoiesis, so storage and use are balanced.

Iron storage

  • Iron is initially stored in ferritin molecules.
  • A single ferritin molecule can store up to 4,000 iron atoms.
  • When excess dietary iron is absorbed, the body responds by producing more ferritin to facilitate iron storage.

Ferritin Storage Molecule

illustration of ferritin storage molecule

Ferritin molecules store thousands of iron atoms within their mineral core. When excess dietary iron is absorbed, the body responds by producing more ferritin to facilitate iron storage.